In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
There are many applications in the field of optics that may require the production of depolarized light, in particular depolarized laser light. For example, a depolarized light output may be used for optical amplification, such as in a Raman amplifier or an erbium doped fibre amplifier (EDFA).
Distributed Raman amplification (DRA) is used in optical networks and can provide advantages of improved reach, improved optical signal-to-noise ratio (OSNR), increased gain bandwidth and flatter gain when compared to other forms of optical amplification.
DRA is achieved by a Raman pump unit injecting high power Raman pump light into a system fibre carrying a signal to be amplified. Typically, a Raman pump unit includes a high power Raman pump laser as a pump source, which provides pump light. The pump light is typically depolarized before being injected into the system fibre as Raman gain in a Raman amplifier can be reduced if the pump light is polarized, since Raman gain is highly polarization dependent.
A number of methods of depolarizing Raman pump light are known.
U.S. Pat. No. 6,977,769 discloses a pump light source device for optical Raman amplification. Combining pump lights through a birefringent material in a polarization beam combiner is disclosed. The composite lightwave output from the combiner is input to a depolarizer comprising birefringent material having a slow axis and a fast axis. The length of the birefringent material of the depolarizer is determined such that the mutual coherence of the pump lights output from the fast axis and the slow axis decreases.
U.S. Pat. No. 7,072,369 discloses a depolarized laser diode module comprising a depolarizer. A polarization maintaining (PM) fibre is arranged at the output side of a laser diode. The depolarizer utilizes a PM fibre for depolarization. The PM fibre output of the laser diode and the PM fibre of the depolarizer are fusion-spliced in such a manner that principal axes of the fibres form an angle of 45 degrees. This type of depolarizer is often termed a fibre Lyot depolarizer. Typically, fibre Lyot depolarizers require up to around 30 m of polarization maintaining fibre to depolarize Raman pump light.
As data speeds get faster, and particularly in coherent systems using 100 Gb/s and above, it is currently expected that, due to data modulation formats, there will be a reduced OSNR budget, meaning techniques for reducing the noise figure in amplifiers is key. As a result, it is envisaged that Raman amplifiers will become more commonly used in systems using 100 Gb/s and above.
Coherent optical systems provide a benefit due to their tolerance of linear impairments, such as Polarisation Mode Dispersion and Chromatic Dispersion. In addition, coherent optical systems allow the use of a more sensitive receiver, thus allowing transmission on similar optical fibre spans as for lower bit rate systems. This is in part supported by use of phase modulation and polarisation multiplexing to produce a modulation scheme with four symbols per bit, which allows the actual baud rate to be four times lower than the transmission speed. However, this provides a restriction on the Polarisation Dependent Loss (PDL) through the spans since both polarisation states are required for the reconstruction of the data. It has been shown that normal levels of PDL impact the OSNR and thus signal detection levels. It is therefore important that any device in the optical span has minimal polarisation dependence. It is known, and described in this document that Raman amplification has strong polarisation dependence and the need to depolarise Raman pump light is well understood. Erbium Doped Fibre amplifiers also have Polarisation Dependent Gain (PDG). In existing systems PDG does not have a big impact on performance, however with coherent transmission the build up of PDG in a series of several in-line amplifiers may begin to impact performance.
Furthermore, there is a desire in the technical field to reduce the size of Raman pump units and EDFA pump units improving their suitability for inclusion in optical devices offering improved functionality, reduced power consumption and/or reduced size. In addition, there is a desire in the technical field to reduce the cost of Raman pump units and increase functionality in EDFA pumps without adding significant cost or complexity to increase their use in optical devices.